1. A modification of the RPC 1 system of A.D. Kelmers, G.D. Novelli & M.P. Stulberg (1965) (J. Biol. Chem. 240, 3979-3983) is described in which the support medium is a Celite of narrow range particle size treated with dichlorodimethylsilane. 2. By using this system an apparently pure preparation of tRNA Cys was isolated from baker9s yeast tRNA. 3. This preparation accepted at least 60% of the theoretical quantity of [3-14C]cysteine in a conventional assay and failed to accept isoleucine, phenylalanine, proline, serine or tyrosine. 4. A theoretical countercurrent-distribution curve calculated by assuming a distribution coefficient K of 2.03 was in excellent agreement with the profiles of E260 and cysteine-acceptor ability after 537 transfers in the 1.85 M-phosphate/formamide/propan-2-ol system of C.M. Connelly & B.P. Doctor (1965) (J. Biol. Chem. 241, 715-719). 5. Chromatography of tRNA Cys on Bio-Gel P100 polyacrylamide beads afforded two components one of which was far less efficient than the other in accepting cysteine. The base compositions of the two were similar.
